The invention relates generally to xerographic copying apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by the use of heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
The commonly utilized approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the other image contacting the fuser roll thereby to affect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereas the toner particles in the image areas of the toner liquify and cause a splitting action in the molten toner resulting in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting", has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene or silicone rubber, the former being known by the tradename Teflon.TM., to which a release agent such as silicone oil is applied, the thickness of the Teflon.TM. being on the order of several mils and the thickness of the oil being less that 1 micron. Silicone based (polydimethylsiloxane) oils, which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon.TM. constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
There are various mechanisms known in the prior art to supply silicone oil to the surface of a heated fuser roll. For small copy volume, a wicking material is impregnated with the oil and is placed in contact with the fuser roll surface. A typical system of this type is disclosed in U.S. Pat. No. 3,964,631 in which a supply roller impregnated with oil is maintained in contact with the fuser roll. Further details of wick construction are disclosed in Xerox Disclosure Journal, Vol. 4, March/April 1975, page 163.
A crucial problem to be solved with any oil dispensing machine is the regulation of the oil distribution so that it is evenly applied to the fuser roll surface. This problem has been addressed in several ways; the Xerox 1075 copier utilizes a metering pump to regulate oil application to a wick applicator. The Xerox 1065 copier utilizes a metering roll system of the type disclosed in the U.S. Pat. No. 3,964,431. These methods are satisfactory for many applications but there remain certain types of operating conditions requiring even more precise oil application mechanism. As one example, the Xerox 2510 large document copier is capable of copying original documents up to 36 inches long, requiring a fuser roll of similar dimensions. Surfaces of this length present exceptional difficulties in maintaining uniform oil application. Several prior art designs address this problem. In U.S. Pat. No. 4,336,766, an oil dispensing device consists of a hollow chamber which supports a first and second wick member, and which has a plurality of apertures for distributing oil at intervals along the length of the fuser roll. U.S. Pat. No. 4,536,076 utilizes a pump to distribute oil to a distribution gallery which communicates with a plurality of drop-forming chambers. The chambers, in turn, dispense oil in droplets onto a wick in contact with the fuser surface. These prior art devices provide acceptable oil distribution, but may still be improved on by further improving the oil application along relatively longer fuser roll lengths.
The present invention is therefore directed to an oil distribution system particularly adapted to maintain uniform oil distribution along a fuser roll surface. More particularly, the present invention is directed toward an apparatus for forming toner images on a copy substrate, said apparatus including a fuser roll, and a release oil distribution assembly, such oil assembly comprising:
a metering chamber including a metering channel having an oil flow surface at a downward inclination of some angle .theta. to the horizontal,
a plurality of enclosed reservoirs located beneath, and in communication with said metering channel, each of said reservoirs having an orifice at the bottom thereof, and
an oil application wick assembly extending along the length of said metering chamber and plurality of reservoirs, said wick assembly adapted to disperse oil by capillary action to the fuser roll surface.